The present invention relates to energy storage devices, including batteries such as lead-acid batteries.
There is growing demand for the development and introduction of vehicles that do not rely almost entirely on fossil fuels, to combat air pollution in urban environments and to reduce the global consumption of limited supplies of the fossil fuels. Such vehicles fall into three main classes: fuel-cell vehicles (FCVs), electric vehicles (EVs), hybrid electric vehicles (HEVs). There are several types of hybrid electric vehicles, namely, micro-, mild-, medium- and full-hybrid. The battery voltage of the hybrid electric vehicles increases in the order: 12 V in micro hybrid, 36 V in mild hybrid, 144 V in medium hybrid and over 200 V in full hybrid. On the other hand, the battery capacity decreases in the order: 50-60 Ah in micro hybrid, 15-20 Ah in mild hybrid, 6-8 Ah in medium hybrid and 6 Ah in full hybrid.
Electric vehicles and hybrid electric vehicles may use a variety of different battery types, including lead-acid batteries. Micro and Mild hybrid electric vehicles may use mainly lead-acid batteries because of reduced cost. Hybrid electric vehicles rely on a combination of an internal combustion engine and a battery for power supply. The hybrid electric vehicles provide some advantages over the existing internal combustion engine cars, including higher use of the electrically generated power, resulting in lower emissions and less fuel consumption.
Whilst there have been many significant advances in the development of new batteries and power networks for vehicles relying at least partly on electric power, the batteries used in these vehicles still suffer from a number of problems.
In all of these batteries, different demands are placed on the battery in terms of the current drawn from and recharged to the battery at various stages during vehicle operation. In the case of vehicle applications, as one example, a high rate of discharge is needed from the battery to enable acceleration in electric vehicles, or acceleration as well as engine cranking in hybrid electric vehicles. A high rate of recharging of the battery is associated with regenerative braking. In such high rate applications (and in high charging applications for the batteries) the battery preferably needs to be able to supply the high rate of discharge over a period of 1 minute or more.
In the situation where lead-acid batteries are utilized, particularly in hybrid electric vehicles, the high rate of battery discharging and recharging results in the formation of a layer of lead sulphate on the surface of the negative plate, and the generation of hydrogen/oxygen at the negative and positive plates. This largely arises as a result of high current demands on the battery. The partial state-of-charge conditions (PSoC) under which these batteries generally operate is 20-100% for electric vehicles, 40-70% for medium and full hybrid electric vehicles, and 70-90% for micro and mild hybrid electric vehicles. This is a high rate partial state-of-charge (HRPSoC). Under simulated HRPSoC duty, such as hybrid electric vehicle operations, the lead-acid batteries fail prematurely mainly due to the progressive accumulation of lead sulphate on the surfaces of the negative plates. This occurs because the lead sulphate cannot be converted efficiently back to sponge lead during charging either from the regenerative braking or from the engine. Eventually, this layer of lead sulphate develops to such an extent that the effective surface area of the plate is reduced markedly, and the plate can no longer deliver the higher current demanded from the automobile. This significantly reduces the potential life span of the battery.
In other technology fields, it would be advantageous to provide alternative battery types that offer improved overall lifespan and performance whilst catering for the different power demands on the battery.
Accordingly, there exists a need for modified batteries, such as lead-acid batteries, that have an improved life span and/or improved overall performance compared to current batteries. There is also a need to identify components of the battery that can be modified to improve performance, in terms of a balance of capacity and lifespan.